In nitric acid solutions and radioactive process liquid wastes generated from reprocessing plants for spent nuclear fuels used in light water reactors and fast reactors, useful FP of rare elements such as platinum group elements of Ru, Rh and Pd are contained. Such rare element FP are high in catalytic activity, and demand for such rare element FP is expected to be increased as electrode materials for fuel cells and as catalysts for producing and purifying fuel hydrogen.
As a technique for separating and recovering such rare element FP from nitric acid solutions containing rare element FP generated from a reprocessing step of spent nuclear fuels used in nuclear power generation facilities including light water reactors and fast reactors, there is a method proposed by the present assignee and described in Japanese Patent Laid-open Specification No. 2003-161798.
In this method, a nitric acid solution containing rare element FP generated from a reprocessing step of spent nuclear fuels is subjected to electrolytic reduction by a constant current electrolysis with Pd2+ or Fe2+ as a catalyst to thereby collectively electrodeposit the rare element FP on an electrode, and the deposits on the electrode are then collectively dissolved by electrolytic oxidation. Thereafter, the solution containing dissolved deposits therein is subjected to electrolytic reduction at a low current density, a medium current density and a high current density, succesively, whereby Ag/Pd group, Se/Te group and Ru/Rh/Tc group are separately deposited and recovered, group by group.
On the other hand, as a method for producing hydrogen to be fuel for the fuel cell power generation which is attracting attention as an energy conversion system clean and excellent in preventing environmental pollution, a reforming method by oxidation of hydrocarbon fuel and a method of electrolyzing alkaline water are widely employed. It is well known that, in both of these methods, the platinum group catalysts are extremely effective.
Although the above-described Japanese Patent Laid-open Specification No. 2003-161798 discloses a concept that the platinum group elements of Ru, Rh and Pd, namely, the rare element FP separated and recovered by using a constant current electrolysis can be effectively utilized as the electrode materials or catalysts for producing and purifying fuel hydrogen for fuel cells, there is taught no specific embodiment for effectively utilizing these FP. Additionally, when the rare element FP thus separated and recovered are intended to be utilized for various applications, it is necessary to adjust the components and to reprocess the forms of the recovered substances in conformity with the intended purposes.
Further, in the hydrogen production method using electrolysis of alkaline water, the stabilization of the supply and cost of the platinum group elements-based catalysts utilized as a catalytic electrode has been an important issue, in order to meet the demand, expected to increase rapidly in the future, for fuel hydrogen for use in the fuel cell power generation.